Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including read only memory (ROM), static random-access memory (SRAM), dynamic random access memory (DRAM), and flash memory.
Conventional DRAM cells are comprised of a switching transistor and an integrated storage capacitor tied to the storage node of the transistor. Charge storage is enhanced by providing appropriate storage capacity in the form of a stacked capacitor or a trench capacitor in parallel with the depletion capacitance of the floating storage node. DRAM cells are volatile and therefore lose data when the power is removed.
DRAMs use one or more arrays of memory cells arranged in rows and columns. Each of the rows of memory cells is activated by a corresponding row line that is selected from a row address. A pair of complementary digit lines are provided for each column of the array and a sense amplifier coupled to the digit lines for each column is enabled responsive to a respective column address. The sense amplifier senses a small voltage differential between the digit lines and amplifies such voltage differential.
Due to finite charge leakage across the depletion layer, the capacitor has to be recharged frequently to ensure data integrity. This is referred to in the art as refreshing and can be accomplished by periodically coupling the memory cells in the row to one of the digit lines after enabling the sense amplifiers. The sense amplifiers then restore the voltage level on the memory cell capacitor to a voltage level corresponding to the stored data bit. The permissible time between refresh cycles without losing data depends on various factors such as rate of charge dissipation in the memory capacitor.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
For code applications, a faster performing, less dense NOR cell is typically employed. For data storage applications, a slower performing, denser NAND cell configuration is typically employed.
SRAM, DRAM, and flash memories typically require relatively large amounts of power. This is due to the volatile memories high current requirement during programming and addressing and/or the requirement for frequent refreshing. Flash memory typically operates by channel hot electron injection that is very power inefficient. Therefore, memory systems that are built on the hierarchy of SRAM/DRAM/flash have a high power requirement and are thus not optimal for portable electronic devices. An additional problem is that flash memory has a limited endurance due to the high programming voltages.
Silicon-oxide-nitride-oxide-silicon (SONOS) type memories have lower power requirements but require higher fields (typically 8-10 W/cm) and are slow to write and erase. Direct Tunnel Memories (DTM) based on ultra-thin tunnel oxide exhibit infinite endurance but memory retention is on the order of 1-10 seconds.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a higher performance and higher endurance non-volatile memory device that can bridge the application gap between DRAM and non-volatile memory devices.